This invention relates to electrophysiological sensors and more particularly to an electrophysiological sensor system which allows the automatic authentication and configuration of the sensor.
When using biomedical sensors to acquire electrophysiological signals for recording and analysis, it is desirable to make certain information concerning the sensor available to the recording and analysis device (monitor). Useful information includes the configuration of electrodes on an electrode sensor, the date of manufacture of the sensor, the identity of the manufacturer and the manufacturer's lot number. A monitor can utilize this information to determine the manner in which to process the acquired data, or even whether to allow the use of the sensor at all (e.g., in the case of an expired sensor).
Such data is entered into the monitor manually by the user, by means of a keyboard, by using a bar code reader to enter data printed on a tag supplied with the sensor, or by various information programs. A simpler method to enter the data is to store the desired information in a memory device of some kind integrated into the sensor itself. The monitor then reads the information automatically, saving the user time and trouble. Various information programs running on the monitor use this information to determine not only the characteristics of the sensor for configuration purposes, but also to verify the viability of a limited life-time sensor, to verify its authenticity and to record various data acquired from the sensor.
The integration of memory devices with medical sensors is well known in the art. In U.S. Pat. No. 5,813,404, Devlin, et al. describe a biopotential electrode connector system in which the configuration of the electrode array is stored in a set of logic lines (jumpers) in the connector of an electrode interface cable. The arrangement described provides for only 8 unique codes, and thus the amount of information which may be stored is severely limited. Also in this invention, the connection of a sensor to the monitor is automatically detected. The monitor incorporates a pulse generator, the pulses of which are used to sense the status of the jumpers (jumpered or open). The determined code is subsequently used to configure the monitor for the particular electrode configuration. This method of automatic sensor detection is suitable for use with passive, hardwired jumpers, but requires a separate pulse generator circuit.
In U.S. Pat. No. 4,580,557, Hertzmann describes the use of coding resistors in the connector of a surgical laser system peripheral output device which serve to identify the particular peripheral device in use. The amount of information that may be stored is again very limited.
In U.S. Pat. No. 5,660,567, Nierlich et aL describe the use of a coding resistor incorporated into a separate module that plugs into the connector of an oximeter probe. Nierlich et aL use the resistor as a mans to code the center wavelength of the red probe emitter. The use of a resistor as a storage device severely limits the amount of information that may be stored.
In U.S. Pat. No. 5,058,558, Kaestle states that the place of application (finger, ear, nose, foot, toe, etc.) of an oximeter has an effect of the accuracy of the measurement. He therefore describes a system for coding the type of sensor (finger sensor, ear sensor, etc.) as a surrogate for the place of application. The code is preferably stored in a coding resistor incorporated in the sensor, which severely limits the amount of information that can be stored. An alternate embodiment would comprise a ROM (read-only memory) or customized integrated circuit, also located in the sensor. While providing more data storage capacity, this embodiment does not provide security for the stored information, nor does it provide the capability for the monitor to store data on the sensor. In addition, the alternate embodiment requires a custom semiconductor device rather than an off-the-shelf device.
In U.S. Pat. No. 4,942,877, Sakai and Hamaguri describe the use of a memory device in or on an oximeter probe; the exact location is not specified. In this probe, the memory device is used to store calibration data relating to the light emitting diode (LED) emitters. An EPROM (electrically programmable, read-only memory) or EEPROM (electrically erasable, programmable, read-only memory) memory device is used. This embodiment does not provide security for the stored information.
In U.S. Pat. No. 4,684,245, Goldring describes the use of a memory chip with a fiberoptic oximeter catheter to store calibration signals. The memory chip is not incorporated into the disposable catheter, but rather into an interface module which can be disconnected from the monitor for transport purposes, so that the calibration data is transported along with the catheter and may be reconnected to a different monitor without necessitating a recalibration.
In U.S. Pat. No. 5,357,953, Merrick, et al describe a similar system for storing calibration data in a separate memory device associated with an invasive optical blood gas analyzer sensor. The blood gas analyzer contains a processor and each disposable blood gas sensor is associated with a self-contained, non-integral non-volatile memory device preferably described as an EEPROM, and alternately as a RAM (random access memory), ROM (read-only memory) or EPROM. The memory device is used to store calibration data specific to the sensor with which it is associated, so that the sensor may be transferred to other blood gas analyzers without recalibration.
In U.S. Pat. No. 4,868,475, Respaut describes the use of a memory device in the transducer system of a scanning mechanical ultrasonic transducer system. The memory device is positioned in the plug of the transducer system connecting the transducer to the associated monitor. The memory device is preferably an EEPROM, but alternately an EPROM or PROM (programmable, read-only memory) and is used to store nonlinearity error information or other information concerning errors in the positioning or scan control for the particular transducer or other calibration information.
In U.S. Pat. No. 5,660,177, Faupel et aL describe an electrode for measuring DC biopotentials that incorporates an addressable chip mounted in either the connector or the cable. This chip, which may be an EEPROM, is designed to be addressed by the processor at a known address. At the start of monitoring, the monitor attempts to interrogate the chip by reading from the preestablished memory location that corresponds to the addressable chip. If the monitor is able to read the memory location corresponding to this address, it proceeds with the measurement program; if it can not read this location, it does not proceed with the measurement program. Faupel further discloses that the monitor may prevent reuse of the electrode by erasing the memory device. Faupel does not specify what information is stored in this memory device or whether the measurement program makes further use of it beyond verifying the presence of an electrode.
While all the devices described above are medical sensors that incorporate some form of memory, they are limited to simply storing calibration and/or configuration data. In contrast, an ideal electrophysiological signal sensor would have the capability to store specific data concerning the sensor itself, such as lot codes, the date of expiration and the sensor serial number, in addition to configuration data It would also encode the identity of the manufacturer and distributor and would encrypt the stored data in order to both protect its integrity and prevent the use of unauthorized substantially equivalent devices. None of the devices described in the patents cited above encrypt the stored data, identify the manufacturer or distributor, use a secure memory device or protect the associated monitor from use with an unauthorized sensor.
The ideal sensor, then, is one that incorporates means for the authentication of its source and the validation of the data stored in its memory. Such a "Smart Sensor" will be part of a sensor authentication and validation system, of which the monitor to which it is connected and which processes the acquired electrophysiological signals is an integral part. The software running in the associated monitor would not only read the data stored on the smart sensor, but also decrypt the data and use it to perform a series of authentications and validations which verify the source of the smart sensor and its physical integrity, while logging its characteristics and various data concerning the conditions of its use. The physical design of the smart sensor, the data stored on it and the accompanying encryption techniques would protect the smart sensor from counterfeiting and provide improved monitoring performance. In addition, such a smart sensor system allows selective functionality to be obtained from a single monitoring system, depending on various configuration codes stored on the smart sensor. Additional functionality may be added after the date of manufacture of the monitor by simply storing different configuration codes on the smart sensor and updating the monitor software.
Another challenge in designing a patient connected sensor which incorporates active electronics in close proximity to a patient is to prevent the application of excess electric current to the patient in both normal and fault conditions